A. Field of the Invention
The present invention relates to metal strips of the type used in spiral pipes and, specifically, to annular (circumferential) pipes and spiral pipes, and to other products formed from such strips.
B. Description of the Related Art
It has long been known that forming metal increases the strength and utility of the metal. The assignee has been a leader in pursuing these effects.
With the recent introduction of construction materials such as polymers, many of the prior strength requirements which were applicable to metals have been lowered to permit greater use of materials of lesser strength. For metal materials to remain competitive, it has been necessary to use metals of lighter thickness and form. However, problems can occur in the manufacture and use of similar products using lighter or less thick metals such as steel and aluminum.
Typically pipe products such as annular pipes and spiral pipes use metal of 18 (0.048″-0.051″) and heavier gauge, otherwise the profiles suffer local buckling and the pipes cannot be successfully manufactured or used. (Note, in this document, “pipe” includes “tube” and vice versa.) Examples of such products, specifically corrugated spiral pipes as well as the process for making such pipes, are disclosed in commonly assigned U.S. Pat. No. 3,247,692, which patent is hereby incorporated by reference.
The buckling usually occurs in the tangent or vertical legs that support the top and/or bottom members which function in part to supply the needed strength to the pipes. Generally speaking, the lighter the metal thickness, the greater the buckling.
In a sinusoidal profile, the tangent area keeps apart the crest and valleys (the arcs) of the sinusoidal profile. If the wall thickness is too light (thin), the tangent gives way to local buckling rather than supporting the profile and allowing it to be formed into a tubular shape.
In the square rib or oblong rib products or similarly profiled products, the local buckling likely will occur in the vertical legs of the ribs.
While conventional profiles such as those disclosed in the incorporated '692 patent are still widely used today, it is desirable to provide new metal forming techniques which alleviate the manufacturing problems associated with present metal thicknesses and profiles, by providing increased strength and utility at reduced metal thicknesses.